1. Field of the Invention
An aspect of the present invention relates to at least one of a fullerene derivative and a method for manufacturing a fullerene derivative.
2. Description of the Related Art
A fullerene discovered in 1985 is a third allotrope of carbon wherein 60 or more carbon atoms are bonded spherically. Attention is being paid to a fullerene represented by C60 or C70 as a new functional material for an electronic component, a drug, a cosmetic, or the like, due to its specific molecular shape.
For a method for synthesizing a fullerene, an arc discharge method, a resistance heating method, a laser evaporation method, a combustion method, a thermal decomposition method, and the like have been known, and soot that contains a fullerene is produced in any of the above manufacturing methods. A fullerene soluble in an organic solvent, such as C60, C70, C76, C78, C82, or C84 is obtained by subjecting such soot to extraction with an organic solvent. Furthermore, it is possible to modify such a fullerene chemically to improve its solubility in an organic solvent or water.
One of promising applications of a fullerene is an organic photoelectric conversion element, such as an organic thin film solar cell or an organic photosensor, and in particular, an organic thin film that is formed in a coating process is actively being studied and developed because it is expected that its production cost is low. [6,6]-phenyl-C61-butyric acid methyl ester (that may be referred to as “[60]PCBM” below) is a representative electron acceptor material soluble in an organic solvent and is frequently used for such applications.
However, in a case where [60]PCBM is combined with an electron donor material such as poly(3-hexylthiophene) to fabricate an organic thin film solar cell, there are problems that it is not possible to provide a sufficiently high open circuit voltage (Voc) because a LUMO level of [60]PCBM is comparatively low and it is not possible to provide a sufficiently high value of short circuit current (Jsc) because a molar absorption coefficient of [60]PCBM in a visible light region is comparatively small.
In recent years, attention is being paid to a fullerene derivative with a substituent introduced at 1,4-positions of a fullerene backbone (that may be referred to as a “1,4-adduct”) as a fullerene derivative capable of solving such problems. In general, a 1,4-adduct is such that extension of a π-electron system on a fullerene backbone is smaller than that of a 1,2-adduct such as [60]PCBM and accordingly its LUMO level is comparatively high, so that it is possible to expect an increase in its open circuit voltage (Voc). Furthermore, a 1,4-adduct has a characteristic absorption peak near 450 nm in a visible light region, so that it is also possible to expect an increase in a short circuit current (Jsc).
In general, as an alkyl group is introduced to a fullerene backbone, its solubility in an organic solvent is improved to increase its compatibility to a coating process. However, an alkyl group is consequently disadvantageous for controlling a LUMO level of a fullerene derivative, because it is difficult to provide a considerable influence on an electronic state of a fullerene backbone. On the other hand, as an aryl group is directly introduced to a fullerene backbone, it is possible to control a LUMO level of a fullerene derivative comparatively easily because it is possible for an electronic state of an aryl group to provide a significant influence on an electronic state of a fullerene backbone although an effect of improving its solubility in an organic solvent is small due to its rigidity. Hence, it is possible to expect that a 1,4-adduct substituted with an alkyl group and an aryl group at a 1-position and a 4-position of a fullerene backbone respectively is a fullerene derivative material with a high solubility and a facilitated control of an electronic state and is excellent as an acceptor material for a photoelectric conversion element, but a practical example of synthesizing such a material is limited.
In Y. Matsuo et al., Chem. Asian J. 8, 121-128 (2013), a fullerene derivative substituted with an aryl group and a silylmethyl group at a 1-position and a 4-position of C60 respectively is synthesized. A synthesis method in Y. Matsuo et al., Chem. Asian J. 8, 121-128 (2013) is a method such that C60 is first reacted with an aryl Grignard reagent, subsequently treated with water to obtain a hydroarylated body as an intermediate and further with a strong alkali to eliminate a hydrogen atom therefrom and thereby produce a fullerene anion, and it is reacted with an alkyl halide group to obtain a target substance. However, this method has a problem that the kind of a substituent capable of being introduced to an alkyl group or an aryl group is limited because a Grignard reagent, a strong alkali, and the like are used. In Y. Matsuo et al., Chem. Asian J. 8, 121-128 (2013), only a fullerene derivative having a substituent could be synthesized wherein the substituent is an alkoxy group, an alkylamino group, an alkyl halide group, or a silyl group.
On the other hand, it becomes clear that a phase separation structure of a current mainstream bulk-hetero-junction-type photoelectric conversion element influences a performance of the element. For a factor influencing a phase separation structure, it is possible to provide a solubility of a material to be used, a compatibility among materials, a solvent to be used for coating, a thermal annealing condition, or the like, and a substituent on a fullerene derivative is important because a solubility or a compatibility with a donor material is significantly influenced thereby. For a representative example of a substituent of a fullerene derivative for influencing a characteristic of a photoelectric conversion element, it is possible to provide an ester structure (an oxycarbonyl group or a carbonyloxy group) for [60]PCBM. K. Moriwaki at al., Tetrahedron 66, 7316-7321 (2010) describes that solubilities of fullerene derivatives with no ester structure, similar to [60]PCBM, in an organic solvent are lower than that of [60]PCBM and characteristics of photoelectric conversion elements using these derivatives are different from one another, and accordingly, suggests that presence or absence of an ester structure in a fullerene derivative influences a phase separation structure for a bulk hetero-junction structure. As described above, a fullerene derivative with an introduced substituent that contains an ester structure is important for controlling a phase separation structure for a bulk hetero-junction structure.
However, as described above, it is not possible for a synthesis method described in Y. Matsuo et al., Chem. Asian J. 8, 121-128 (2013) to synthesize a fullerene derivative having a highly reactive substituent that contains an ester structure. That is, it has not been possible to realize synthesis of a 1,4-adduct that is substituted with an alkyl group for ensuring a solubility in an organic solvent and an aryl group for facilitating a control of a LUMO level and further contains an ester structure in a portion of those groups to control a phase separation structure appropriately.